The US-based Human Rights Watch has launched a stinging attack on Sri Lanka's judicial system after four men were acquitted of killing 27 Tamils.
The camp at Bindunuwewa was destroyed after the attack
It said the acquittals revealed "a shocking failure of the police and judicial system to find justice for the dead and injured".
The Tamils died when locals stormed their detention centre in 2000.
Last week, the Supreme Court said the defendants' guilt had not been proved beyond reasonable doubt.
A Sri Lankan presidential spokesman told the BBC the case had gone through due process, and said he was not prepared to comment on a judicial matter. The judicial authorities could not immediately be reached for comment.
Human Rights Watch said the case showed crimes committed against alleged Tamil Tiger members were not being addressed.
It called for a new investigation to be launched immediately to identify those responsible for the killings.
Brad Adams, Asia director of Human Rights Watch, said if there was no new investigation "it will only further distance aggrieved Tamils".
The Supreme Court had itself severely criticised the role of the police in the massacre.
The 25 October 2000 attack at Bindunuwewa in central Sri Lanka was internationally condemned.
As well as the 27 who died, 14 others were seriously injured in the attack.
Local residents assaulted the inmates with clubs and knives after reports they had taken a security guard hostage. Some victims were said to have been burned alive.
The four acquitted men had been sentenced to death by the Colombo High Court in July 2003 but appealed.
The Tamil Tigers have fought a two-decade armed campaign for a homeland in the north and east. More than 60,000 people have died.